The invention generally relates to laser sources, and particularly relates to laser sources for providing dual frequency outputs fields for applications such as coherent anti-Stokes Raman scattering microscopy (CARS).
CARS microscopy allows imaging of chemical and biological samples by using molecular vibrations as a contrast mechanism. Conventional CARS microscopy is a third order non-linear process and uses two laser fields: a pump electromagnetic field with a center frequency at ωp and a Stokes electromagnetic field with a center frequency at ωs. The pump and Stokes fields interact with a sample and generate a coherent anti-Stokes field having a frequency of ωAS=2ωp−ωS. When the Raman shift of ωp−ωs is tuned to be resonant at a given vibrational mode, an enhanced CARS signal is observed at the anti-Stokes frequency ωAS. For example, U.S. Pat. Nos. 6,809,814 and 6,798,507 (the disclosures of which are hereby incorporated by reference) disclose CARS microscopy systems employing epi-detected CARS signals and polarization CARS signals respectively.
Conventional laser sources for CARS microscopy provide broad tuning ranges using, for example, two mode-locked titanium sapphire (Ti:Sapphire) lasers that are electronically synchronized. Although such electronically locked sources provide broad tuning ranges, high spectral resolution, and could function in an ultra short pulse regime for a variety of applications, such sources also exhibit timing jitter due to difficulties of electronic synchronization, and are characterized as being relatively complex and expensive.
Another conventional laser source for CARS microscopy involves a mode-locked Nd:YVO4 pump laser (1064 nm) and a synchronously pumped optical parametric oscillator (OPO), which provides high spectral resolution (about 1 cm−1) and has no timing jitter. The OPO is intra-cavity doubled to provide a pump beam for CARS microscopy. A small fraction of the Nd:YVO4 laser provides the Stokes beam. Such laser sources, however, involve narrow tuning ranges that may include gaps, multiple crystals are sometimes required to cover the entire Raman spectral range, the spatial mode quality may be less than is desired because of the intra-cavity doubling, and the pump and stokes wavelengths may be less than desired.
It is desirable to provide a laser source for CARS microscopy that provides continuous tunability for regions of Raman frequencies (e.g., from 500 cm−1 to 3500 cm−1 which covers most of the resonances in certain applications such as life tissue in biological and molecular applications. Other desirable characteristics include high spectral resolution with wavelength of only a few cm−1, low intensity noise (high stability), very good synchronization of the pump and Stokes pulses—high repetition rate for high frame rate imaging, and favorable wavelengths to avoid or reduce photo-damage and to attain higher penetration depths. The system, therefore, provides greater flexibility and functionality for CARS in that operating wavelengths may vary from 900-1300 nm in certain systems of the invention. This significantly impacts capabilities of the system, including penetration depth and non-invasiveness for cells and optically dense soft tissue. The source should also be compact, easy and reliable to use, and low in cost.
Because CARS is a non-linear analytical process for biological applications, high peak power with low average power is required to generate the required signal without degrading the biological sample. Picosecond or femtosecond pulses at high power are therefore desired. U.S. Pat. No. 5,017,806 discloses a synchronously pumped optical parametric oscillator that provides a femtosecond pulse train and is pumped by a femtosecond pulsed dye laser. Although the system is disclosed to be suitable for use with a broad tuning range, dye lasers are considered to be relatively unstable during use and too unreliable over time for use in vibrational biological analyses.
Mode locked titanium:sapphire (Ti:Sa) lasers are also known to be used to pump optical parametric oscillators, but such systems also do not provide an output of a sufficiently broad spectral range that may cover the entire spectral range for vibrational analysis of biological samples. See for example, P. E. Powers, R. J. Ellington and W. S. Pelouch, Recent Advances of the Ti:sapphire-pumped high-repetition-rate femtosecond optical parametric oscillator, J. Opt. Soc. Am., vol. 10, No. 11, November 1998.
It is also known that a non-linear crystal pumped by a continuous wavelength (CW) laser at a frequency in the visible range (about 400-about 700 nm) may be provided using a solid state laser. See for example, R. G. Batchko, D. R. Weise, T. Plettner, G. D. Miller, M. M. Fejer, and R. L. Byer, Continuous-wave 532-nm-Pumped Singly Resonant Optical Parametric Oscillator Based on Periodically Poled Lithium Niobate, Optics Letters, Vol. 23, No. 3 (Feb. 1, 1998). Such as system, however, provides a CW signal that is much too high in average power to be used for vibrational biological analyses. Optical parametric oscillators synchronously pumped by picosecond Nd:YLF lasers at frequencies in the visible range have also been reported. See for example, P. Heinz, A. Seilmeier and A. Piskarskas, Picosecond Nd:YLF laser-multipass amplifier source pumped by pulsed diodes for the operation of powerful OPOs, Optics Communications, v. 136, pp. 433-436 (1997). The low repetition rates of these systems, however, are not suitable for vibrational biological analysis.
There is a need, therefore, for a more efficient and economical system for providing illumination in a vibrational biological analysis system such as a CARS microscopy or spectroscopy system.